Base station system and communication apparatus

ABSTRACT

A base station system according to an aspect of the invention includes a baseband unit (BBU) and a radio frequency (RF) unit (RFU) connected to the BBU via a communication line. The RFU (BBU) measures the frequencies of occurrence of sampled values indicated by sampled data that is a digital signal corresponding to a baseband signal to be transmitted to the BBU (RFU), and generates a frequency distribution representing a relationship between the sampled value having occurred and the frequency of occurrence. Furthermore, the RFU (BBU) determines a plurality of thresholds for compressing the sampled data, which are used for quantization processing of the sampled value, in accordance with the generated frequency distribution, and compresses the sampled data by the quantization processing using the plurality of thresholds.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Patent ApplicationNo. PCT/JP2013/003392 filed on May 29, 2013, the entire content of whichis incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a base station system and acommunication apparatus and, more particularly, to processing ofcompressing and decompressing a baseband signal transmitted via acommunication line between a baseband unit and a radio frequency unit inthe base station system.

BACKGROUND ART

A base station installed in a radio access network is generally formedfrom a baseband unit (BBU) for executing baseband processing and thelike and an radio frequency (RF) unit (RFU) for executing RF processingand the like. An arrangement in which a BBU is integrated with an RFU isknown as the arrangement of such a base station.

To the contrary, in recent years, in a radio access network such as LTE(Long Term Evolution), a base station (base station system) in which itis possible to install a BBU and an RFU at different locations and whichis capable of connecting a plurality of RFUs to one BBU via an opticalinterface is becoming widespread. In such a base station, for example,CPRI (Common Public Radio Interface) has been stipulated as the standardof an interface for connecting an RFU and a BBU. For example, the BBUand RFU transmit digitized baseband signals to each other via an opticalinterface complying with CPRI. Note that the RFU may also be referred toas a remote radio head (RRH).

In a CPRI line for connecting a BBU and an RFU, due to an enlargement ofthe bandwidth of a base station, an implementation of multiband, anincrease in multiplex number of MIMO, and the like, a large amount oftraffic occurs and a necessary transmission capacity can suddenlyincrease. A large amount of traffic can be accommodated by laying down anumber of optical fibers between the BBU and the RFU but theconstruction of the optical fibers requires a high cost. Although it canbe considered to multiplex a number of CPRI lines by wavelength divisionmultiplexing (WDM), this increases the apparatus cost for an opticaltransceiver and the like.

In view of the above, a technique of compressing a baseband signal andtransmitting the compressed signal between the BBU and the RFU has beenconsidered to accommodate a large amount of traffic by a CPRI line whilesuppressing an increase in cost. PTL 1 proposes some methods forcompressing digitized baseband signal samples to be transmitted betweena BBU and an RFU. More specifically, compression by Huffman coding,compression by calculation of a primary or higher-order differencebetween baseband signal samples and coding of the difference,compression based on at least one of a sampling rate, sample width,bandwidth, and modulation type, and the like are proposed. For example,in compression by Huffman coding, the frequencies of occurrence ofsampled values are obtained in advance using a series of sampled valuesof a baseband signal, and sample compression is performed by assigning ashorter code to a sampled value as its frequency of occurrence ishigher.

CITATION LIST Patent Literature PTL 1: Japanese Patent Laid-Open No.2011-526095 SUMMARY OF INVENTION Technical Problem

In the technique described in PTL 1, however, the following problemsarise. For example, when performing data compression by Huffman codingas described above, if the frequency distribution of sampled valuescorresponding to a baseband signal temporally varies, a data compressionratio also temporally varies. As exemplified in FIG. 10, the averagepower of a baseband signal transmitted between a BBU and an RFU varieson the order of several hundred milliseconds, and accordingly, thefrequency distribution of sampled values corresponding to the basebandsignal also varies on the same order. Consequently, the data compressionratio in compression by Huffman coding temporally varies. As a result,it may be impossible to ensure a sufficient data compression ratio whencompressing the baseband signal transmitted between the BBU and the RFU,and to achieve a required data compression ratio (for example, 50%).

Alternatively, when performing data compression by calculation of aprimary or higher-order difference between baseband signal samples andcoding of the difference, it may be impossible, due to a temporalvariation in baseband signal, to suppress the number of bits requiredfor coding of the difference between sampled values to that whichachieves a required data compression ratio. In this case, it may beimpossible to achieve the required data compression ratio whilesuppressing degradation in signal quality along with compression.

The present invention has been made in consideration of theabove-described problems, and provides a technique of making it possibleto compress, at a higher data compression ratio, a baseband signaltransmitted between a BBU and an RFU via a communication line in a basestation system while suppressing degradation in signal quality alongwith compression.

Solution to Problem

According to one aspect of the present invention, there is provided abase station system comprising a baseband unit and a radio frequency(RF) unit connected to the baseband unit via a communication line, theRF unit comprising: a generation unit configured to generate, as sampleddata, a digital signal corresponding to a baseband signal to betransmitted to the baseband unit; a measurement unit configured tomeasure frequencies of occurrence of sampled values indicated by thesampled data generated by the generation unit, and generate a frequencydistribution representing a relationship between a sampled value havingoccurred and a frequency of occurrence; a determination unit configuredto determine a plurality of thresholds for compressing the sampled data,which are used for quantization processing of a sampled value, inaccordance with the frequency distribution generated by the measurementunit; and a compression unit configured to compress the sampled data bythe quantization processing using the plurality of thresholds determinedby the determination unit to generate compressed data to be transmittedto the baseband unit.

According to another aspect of the present invention, there is provideda base station system comprising a baseband unit and a radio frequency(RF) unit connected to the baseband unit via a communication line, thebaseband unit comprising: a generation unit configured to generate, assampled data, a digital signal corresponding to a baseband signal to betransmitted to the RF unit; a measurement unit configured to measurefrequencies of occurrence of sampled values indicated by the sampleddata generated by the generation unit, and generate a frequencydistribution representing a relationship between a sampled value havingoccurred and a frequency of occurrence; a determination unit configuredto determine a plurality of thresholds for compressing the sampled data,which are used for quantization processing of a sampled value, inaccordance with the frequency distribution generated by the measurementunit; and a compression unit configured to compress the sampled data bythe quantization processing using the plurality of thresholds determinedby the determination unit to generate compressed data to be transmittedto the RF unit.

According to one aspect of the present invention, there is provided acommunication apparatus for communicating with an opposing apparatusconnected via a communication line in a base station system, comprising:a generation unit configured to generate, as sampled data, a digitalsignal corresponding to a baseband signal to be transmitted to theopposing apparatus; a measurement unit configured to measure frequenciesof occurrence of sampled values indicated by the sampled data generatedby the generation unit, and generate a frequency distributionrepresenting a relationship between a sampled value having occurred anda frequency of occurrence; a determination unit configured to determinea plurality of thresholds for compressing the sampled data, which areused for quantization processing of a sampled value, in accordance withthe frequency distribution generated by the measurement unit; and acompression unit configured to compress the sampled data by thequantization processing using the plurality of thresholds determined bythe determination unit to generate compressed data to be transmitted tothe opposing apparatus.

Advantageous Effects of Invention

According to the present invention, it is possible to compress, at ahigher data compression ratio, a baseband signal transmitted between aBBU and an RFU via a communication line in a base station system whilesuppressing degradation in signal quality along with compression.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings. Note that the same reference numerals denote thesame or like components throughout the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments of theinvention and, together with the description, serve to explain theprinciples of the present invention.

FIG. 1 is a block diagram showing an example of the configuration of abase station system;

FIG. 2 is a block diagram showing examples of the arrangements of an RFUand a BBU in the base station system;

FIG. 3 is a block diagram showing an example of the arrangement of thecompression unit of each of the BBU and RFU;

FIG. 4 is a block diagram showing an example of the arrangement of thedecompression unit of each of the BBU and RFU;

FIG. 5 is a graph showing an example of a frequency distributiongenerated by a frequency distribution generation unit;

FIG. 6 is a view schematically showing a quantization thresholddetermination process based on the frequency distribution;

FIG. 7 is a view schematically showing the quantization thresholddetermination process based on the frequency distribution;

FIG. 8 is a table showing an example of a conversion table used forcompression processing;

FIG. 9 is a view schematically showing a quantization thresholddetermination process based on a cumulative distribution function; and

FIG. 10 is a view showing an example of the frequency distribution ofsampled values corresponding to a baseband signal.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present invention will be described belowwith reference to the accompanying drawings. Note that componentsunnecessary for the description of the embodiments are omitted from theaccompanying drawings.

First Embodiment

The first embodiment of the present invention will be described withreference to FIGS. 1 to 8.

<Configuration of Base Station System>

FIG. 1 is a block diagram showing an example of the configuration of abase station system 100 according to this embodiment. In thisembodiment, the base station system 100 includes a central office (CO)110 connected to a core network, and a plurality of RF units (RFUs) 120a and 120 b each for executing radio frequency (RF) processing and thelike in radio communication. A baseband unit (BBU) 130 for executingbaseband processing and the like in radio communication is provided inthe central office 110. The RFUs 120 a and 120 b form service areas 140a and 140 b for radio communication, respectively. Each of the RFUs 120a and 120 b is connected to an antenna for cellular communication, andperforms, via the antenna, radio communication with a user terminal suchas a mobile phone existing in the service area formed by itself.

In this embodiment, as shown in FIG. 1, the BBU is centralized in thecentral office 110, and shared by the plurality of RFUs, instead ofindividually providing the BBU for each of the plurality of RFUs. Anarbitrary number of RFUs are connectable to the BBU 130 via acommunication line 150. In the base station system 100, the two RFUs 120a and 120 b are connected to the BBU 130, as an example. Note that theRFUs 120 a and 120 b have the same arrangement and, in the followingexplanation, an RFU 120 indicates each of the RFUs 120 a and 120 b.

The communication line 150 is a front-haul line connecting the BBU 130and the RFU 120 and, in this embodiment, is formed as an optical line(CPRI line) complying with CPRI. The BBU 130 and the RFU 120 transmitdigitized baseband signals to each other via the communication line 150.

FIG. 2 is a block diagram showing examples of the arrangements of theRFU 120 and the BBU 130 in the base station system 100 according to thisembodiment. The RFU 120 is connected to an antenna 201, and includes areception unit (Rx) 202, an analog-to-digital converter (ADC) 203, acompression unit 204, a decompression unit 205, a digital-to-analogconverter (DAC) 206, and a transmission unit (Tx) 207. The BBU 130includes a decompression unit 211, a signal processing unit 212, and acompression unit 213. The compression unit 204 on the side of the RFU120 is connected to the decompression unit 211 on the side of the BBU130 via the communication line 150. The compression unit 213 on the sideof the BBU 130 is connected to the decompression unit 205 on the side ofthe RFU 120 via the communication line 150. The arrangement of each ofthe compression units 204 and 213 and that of each of the decompressionunits 205 and 211 will be described later with reference to FIGS. 3 and4. Note that in the base station system 100, the compression units 204and 213, decompression units 211 and 213, and signal processing unit 212individually execute processing (to be described later) for data of eachof an in-phase (I) channel component and quadrature (Q) channelcomponent of the baseband signal (sampled data).

(Signal Transmission in Uplink Direction)

Signal transmission in an uplink direction from the RFU 120 to the BBU130 in the base station system 100 will be explained. In the RFU 120,the antenna 201 receives radio signals transmitted from one or more userterminals. The radio signal received by the antenna 201 is, after beingconverted into an analog signal (analog electric signal), down-convertedfrom an RF to a baseband frequency by the Rx 202. This generates abaseband signal containing an I channel component and a Q channelcomponent. The baseband signal output from the Rx 202 is input to theADC 203.

The ADC 203 converts each of the I channel component and Q channelcomponent of the input baseband signal from an analog signal to adigital signal by sampling processing and quantization processing, andoutputs the converted signal as sampled data. The sampled data includesI channel data and Q channel data. Note that the ADC 203 performssampling processing at a sampling frequency Fs, and performsquantization processing with a sample width (the number of quantizationbits) of N bits. The sampled data output from the ADC 203 is input tothe compression unit 204. In this manner, the Rx 202 and the ADC 203generate, as sampled data, a digital signal corresponding to thebaseband signal to be transmitted to the BBU 130.

The compression unit 204 executes, for the input sampled data,compression processing of converting (compressing) sampled data of Nbits per sample into sampled data (compressed data) of M bits (M<N) persample. That is, the compression unit 204 executes compressionprocessing of a compression ratio (data compression ratio) M/N. TheM-bit compressed data output from the compression unit 204 istransmitted to the BBU 130 via the communication line 150.

In the BBU 130, the compressed data received from the RFU 120 via thecommunication line 150 is first input to the decompression unit 211. Thedecompression unit 211 executes, for the input compressed data,decompression processing of converting (decompressing) sampled data of Mbits per sample into sampled data of N bits per sample. The N-bitsampled data output from the decompression unit 211 is output to thesignal processing unit 212. The signal processing unit 212 extracts userdata by performing predetermined baseband signal processing(demodulation, error correction, and the like) for the input sampleddata (baseband signal).

(Signal Transmission in Downlink Direction)

Signal transmission in a downlink direction from the BBU 130 to the RFU120 in the base station system 100 will now be explained. In the BBU130, the signal processing unit 212 sets one or more user terminals asdestinations, generates, as sampled data, a digital signal correspondingto a baseband signal to be transmitted to the RFU 120, and outputs thesampled data to the compression unit 213. Similarly to the compressionunit 204, the compression unit 213 executes, for the input sampled data,compression processing of converting (compressing) sampled data of Nbits per sample into sampled data (compressed data) of M bits (M<N) persample. The M-bit compressed data output from the compression unit 213is transmitted to the RFU 120 via the communication line 150.

In the RFU 120, the compressed data received from the BBU 130 via thecommunication line 150 is first input to the decompression unit 205.Similarly to the decompression unit 211, the decompression unit 205executes, for the input compressed data, decompression processing ofconverting (decompressing) sampled data of M bits per sample intosampled data of N bits per sample. The N-bit sampled data output fromthe decompression unit 211 is output to the DAC 206. The DAC 206respectively converts the I channel component data and Q channelcomponent data of the input sampled data into analog signals, andoutputs the converted signals to the Tx 207. The Tx 207 up-converts theinput analog signals of the I channel and Q channel from the basebandfrequency to the RF, and outputs the up-converted analog signals to theantenna 201. The analog signals of the RF are transmitted from theantenna 201 as radio signals.

Note that in this embodiment, in signal transmission in the uplinkdirection, the RFU 120 is an example of a communication apparatus forcommunicating with an opposing apparatus connected via the communicationline 150, and the BBU 130 is an example of the opposing apparatus. Onthe other hand, in signal transmission in the downlink direction, theBBU 130 is an example of a communication apparatus for communicatingwith an opposing apparatus connected via the communication line 150, andthe RFU 120 is an example of the opposing apparatus.

In this embodiment, in the compression processing by the compressionunit 204 or 213, lossy compression is performed for the digitizedbaseband signal (sampled data) to be transmitted via the communicationline 150 so as to increase the compression ratio as much as possible.That is, if M-bit (M<N) sampled data compressed from N-bit sampled datais decompressed to N-bit sampled data by the decompression unit 205 or211, data different from the N-bit sampled data before compression canbe obtained. This can degrade the quality of the baseband signaltransmitted between the RFU 120 and the BBU 130.

This embodiment has as its feature to perform compression processing bynon-uniform (non-linear) quantization processing in accordance with thefrequency distribution of the sampled values of the baseband signal, inorder to achieve a high compression ratio even under an environment inwhich the baseband signal temporally varies, while suppressingdegradation in signal quality along with compression.

<Compression Unit>

FIG. 3 is a block diagram showing an example of the arrangement of thecompression unit 204 of the RFU 120 according to this embodiment. Notethat the compression unit 213 of the BBU 130 has the same arrangement.The compression unit 204 includes a buffer 311, a frequency distributiongeneration unit 312, a quantization threshold determination unit 313, acoder 314, and a compression information addition unit 315. The sampleddata input to the compression unit 204 is saved in the buffer 311 andinput to the frequency distribution generation unit 312.

Generation of Frequency Distribution

The frequency distribution generation unit 312 measures, using the N-bitsampled data, the frequencies of occurrence of sampled values indicatedby the sampled data, and generates a frequency distribution representingthe relationship between the sampled value having occurred and itsfrequency of occurrence. The frequency distribution generation unit 312generates a frequency distribution for each of I channel data and Qchannel data included in the sampled data. Note that the frequencydistribution generation unit 312 generates a frequency distributionusing the sampled data (a series of sampled data) corresponding to apredetermined number of samples. Furthermore, the frequency distributiongeneration unit 312 can update the frequency distribution every time thesampled data of one sample is input.

FIG. 5 is a graph showing an example of the frequency distributiongenerated by the frequency distribution generation unit 312. FIG. 5shows a frequency distribution generated using the I channel data of a16-bit (N=16) digital signal obtained by converting an actual uplink LTEsignal received for 5 sec. Note that the abscissa represents the sampledvalue (amplitude) of an I channel, and the ordinate represents aprobability density corresponding to the frequency of occurrence. Thefrequency distribution generated by the frequency distributiongeneration unit 312 depends on the statistical features of the basebandsignal to be transmitted to the BBU 130. The frequency distributionshown in FIG. 5 indicates a normal distribution 500, and it isunderstood that the obtained frequency distribution can be generallyapproximated by the normal distribution 500.

If it is known in advance that the baseband signal (sampled data) to beprocessed by the frequency distribution generation unit 312 can beapproximated by the normal distribution, as shown in FIG. 5, it ispossible to readily generate a frequency distribution by calculating amean μ and a standard deviation σ of the frequencies of occurrence ofthe sampled values. In this case, the frequency distribution generationunit 312 measures the frequencies of occurrence of the sampled valuesusing the input series of sampled data, and calculates the mean μ andstandard deviation σ(or variance σ²) of the frequencies of occurrencebased on the measurement result. Furthermore, the frequency distributiongeneration unit 312 generates, as a frequency distribution, a normaldistribution N(μ, σ²) determined by the calculated mean μ and standarddeviation σ.

(Determination of Quantization Threshold)

The frequency distribution generated by the frequency distributiongeneration unit 312 is input to the quantization threshold determinationunit 313. In accordance with the input frequency distribution, thequantization threshold determination unit 313 determines a plurality ofthresholds for compressing (quantizing) the sampled data by the coder314, which are used for sampled value quantization processing. Morespecifically, for example, to compress (quantize) the N-bit sampled datato the M-bit sampled data, the quantization threshold determination unit313 generates 2^(M) thresholds (to also be referred to as “quantizationthresholds” hereinafter) to be compared with the sampled values at thetime of compression.

FIGS. 6 and 7 are views schematically showing a quantization thresholddetermination process based on the frequency distribution by thequantization threshold determination unit 313. FIG. 6 shows a case inwhich the frequency distribution generation unit 312 generates afrequency distribution 600 by approximation using the normaldistribution. Note that in FIG. 6, the abscissa represents each sampledvalue (amplitude) of the N-bit sampled data by a decimal number and theordinate represents the frequency of occurrence of each sampled value.As shown in FIG. 6, based on the frequency distribution 600, thequantization threshold determination unit 313 determines thresholds 610the number (2^(M)) of which corresponds to the compression ratio. Thethresholds 610 can be determined as shown in FIG. 7.

FIG. 7 shows quantization thresholds for obtaining N-bit (6-bit) sampleddata, which are used for quantization by the ADC 203, and quantizationthresholds for obtaining M-bit (3-bit) sampled data, which are used forcompression (quantization) by the compression unit 204. Note that theADC 203 performs N-bit quantization using the plurality of thresholdswith equal intervals, thereby generating N-bit sampled data. On theother hand, the compression unit 204 converts (compresses)N-bit sampleddata into M-bit sampled data using the plurality of thresholds withunequal intervals, which are determined in accordance with the frequencydistribution generated by the frequency distribution generation unit312.

The quantization thresholds used in compression by the compression unit204 can be determined in accordance with the frequency distribution, asfollows. More specifically, as shown in FIG. 7, the quantizationthreshold determination unit 313 may determine the plurality ofthresholds so that, for each of the plurality of sections obtained bydividing, using the plurality of thresholds, a range within which thesampled values can fall, the width (quantization step width orquantization step size) of the section becomes narrower as the frequencyof occurrence of the sampled value included in the section is higher. Tothe contrary, the quantization threshold determination unit 313 maydetermine the plurality of thresholds so that, for each of the pluralityof sections obtained by dividing, using the plurality of thresholds, therange within which the sampled values can fall, the width of the sectionbecomes wider as the frequency of occurrence of the sampled valueincluded in the section is lower.

Determination of a plurality of quantization thresholds by thequantization threshold determination unit 313 can be implemented usingthe frequency distribution by, for example, the following method. Thatis, for each of the plurality of sections obtained by dividing, usingthe plurality of thresholds, the range within which the sampled valuescan fall, the integrated value of the frequencies of occurrence ofsampled values included in the section is calculated. Furthermore, aplurality of quantization thresholds may be determined so that thecalculated integrated values for the plurality of respective sectionsbecome equal to each other (as much as possible). More specifically, theplurality of quantization thresholds may be determined so that a valueobtained by integrating the frequencies of occurrence of sampled valuesin a section (quantization step) between a j-th quantization thresholdand a (j+1)-th quantization threshold becomes equal for all of j=0, . .. , 2^(M)−1 as much as possible.

In addition, the quantization threshold determination unit 313determines, based on the plurality of determined quantizationthresholds, the correspondence between an output of the ADC 203 and thatof the coder 314. FIG. 8 is a table showing an example of thecorrespondence between an output of the ADC 203 and that of the coder314 when N=6 and M=3. The quantization threshold determination unit 313assigns one output of the coder 314 to each section of sampled valuesobtained by dividing, using the quantization thresholds determined asdescribed above, the range within which the sampled values (amplitudes)can fall. For example, “011” is assigned, as an output of the coder 314,to a section of sampled values (amplitudes) of 15 to 31 (outputs“001111” to “011111” of the ADC 203). The quantization thresholddetermination unit 313 outputs, to the coder 314 and the compressioninformation addition unit 315, data indicating a conversion table forassociating N-bit sampled data with M-bit sampled data, as shown in FIG.8.

(Compression Processing)

The series of sampled data input to the compression unit 204 is saved(buffered) in the buffer 311 until the quantization thresholddetermination unit 313 determines quantization thresholds, and generatesthe conversion table as shown in FIG. 8. After the conversion tablegenerated by the quantization threshold determination unit 313 is input,the coder 314 sequentially reads out N-bit sampled data from the buffer311, and performs compression processing for the readout sampled data.In this way, the coder 314 applies, to the series of sampled data usedby the frequency distribution generation unit 312 to generate thefrequency distribution, compression processing using the quantizationthresholds determined in accordance with the frequency distribution.

The coder 314 generates compressed data to be transmitted to the BBU 130by compressing the sampled data by quantization processing using theplurality of quantization thresholds determined by the quantizationthreshold determination unit 313. More specifically, the coder 314converts the N-bit sampled data (the output of the ADC 203) read outfrom the buffer 311 into the associated M-bit sampled data by referringto the conversion table output from the quantization thresholddetermination unit 313. With this processing, the coder 314 compressesthe N-bit sampled data to the M-bit sampled data, and outputs thecompressed data.

(Transmission Processing)

The sampled data (compressed data) output from the coder 314 is input tothe compression information addition unit 315. The compressioninformation addition unit 315 adds, to the input compressed data,control information (compression information) to be used indecompression processing by the decompression unit 211 of the BBU 130 onthe reception side, and transmits the resultant data to the BBU 130 viathe communication line 150. The control information includes informationindicating the plurality of quantization thresholds determined by thequantization threshold determination unit 313, and may include, forexample, the conversion table as shown in FIG. 8. Alternatively, thecompression unit 204 and the decompression unit 211 may hold in advancea plurality of conversion tables corresponding to a plurality ofdifferent frequency distributions, and the coder 314 may performcompression processing using the conversion table corresponding to thefrequency distribution generated by the frequency distributiongeneration unit 312. In this case, the control information transmittedby the compression information addition unit 315 may include informationindicating the conversion table used. In addition, the compressioninformation addition unit 315 may transmit, as the control informationtransmitted to the BBU 130, the mean μ and standard deviation σcalculated by the frequency distribution generation unit 312 to the BBU130.

<Decompression Unit>

FIG. 4 is a block diagram showing an example of the arrangement of thedecompression unit 211 of the BBU 130 according to this embodiment. Notethat the decompression unit 205 of the RFU 120 has the same arrangement.The decompression unit 211 includes a compression information extractionunit 411, a quantization threshold determination unit 412, a buffer, anda decoder 414. The data which has been received from the RFU 120 via thecommunication line 150 and input to the decompression unit 211 is inputto the compression information extraction unit 411.

The compression information extraction unit 411 extracts, from the inputdata, control information (compression information) added to thecompressed data by the compression information addition unit 315, andoutputs the control information to the quantization thresholddetermination unit 412. Furthermore, the compression informationextraction unit 411 saves, in the buffer 413, the compressed dataincluded in the input data. Based on the control information input fromthe compression information extraction unit 411, the quantizationthreshold determination unit 412 determines a plurality of quantizationthresholds to be used in decompression processing by the decoder 414.For example, the quantization threshold determination unit 412 mayextract the conversion table as shown in FIG. 8 from the controlinformation, and output the conversion table to the decoder 414.

The decoder 414 sequentially reads out the M-bit compressed data(sampled data) from the buffer 413, and performs decompressionprocessing of decompressing the compressed data using the conversiontable input from the quantization threshold determination unit 412. Morespecifically, the decoder 414 performs decompression processing byconverting the M-bit compressed data into N-bit sampled data. Forexample, the decoder 414 outputs, as decompressed sampled data, one of aplurality of N-bit outputs of the ADC, which corresponds to the M-bitcompressed data (the output of the coder).

As described above, according to this embodiment, since compressionprocessing is performed so that a quantization step corresponding to asampled value becomes narrower as the frequency of occurrence of thesampled value is higher, it is possible to reduce an error between thesampled values before and after compression. On the other hand, sampleddata for which an error between sampled values before and aftercompression becomes large is limited to sampled data for which thefrequency of occurrence of a sampled value is low. Consequently, whenapplying compression processing to N-bit sampled data, it is possible tomore accurately represent the sampled data by M-bit (M<N) sampled data.According to this embodiment, it is therefore possible to compress thebaseband signal at a high compression ratio while suppressingdegradation (that is, quantization noise) in signal quality along withcompression processing, by adaptively controlling quantizationthresholds to be used for data compression in accordance with thefrequency distribution.

Note that the frequency distribution generated by the frequencydistribution generation unit 312 can be updated by one sample atminimum. Therefore, a processing delay amount caused by compressionprocessing and decompression processing can be suppressed to a smallamount (for example, 100 μsec or less).

Second Embodiment

The second embodiment of the present invention will be described next.In this embodiment, the frequency distribution of sampled data isapproximated by a normal distribution, and a plurality of quantizationthresholds for compressing the sampled data is determined in accordancewith the normal distribution. Especially, this embodiment has as itsfeature to calculate the cumulative distribution function of thefrequency distribution (normal distribution), and determine a pluralityof quantization thresholds based on the calculated cumulativedistribution function. Note that the configuration (FIGS. 1 to 4) of abase station system 100 (a BBU 130 and RFUs 120) according to thisembodiment is the same as in the first embodiment. The operation of thebase station system 100 different from the first embodiment will bedescribed below exemplifying baseband signal transmission in an uplinkdirection from the RFU 120 to the BBU 130.

<Compression Unit>

In a compression unit 204 of the RFU 120, a frequency distributiongeneration unit 312 measures the frequencies of occurrence of sampledvalues indicated by input sampled data, and calculates a mean μ andstandard deviation σ of them. The frequency distribution generation unit312 generates, as a frequency distribution, a normal distribution N(μ,σ²) determined by the calculated mean μ and standard deviation σ. Notethat the frequency distribution generation unit 312 generates afrequency distribution for each of I channel data and Q channel data,similarly to the first embodiment. A probability density function f(x)of the normal distribution N(μ, σ²) is given by:

$\begin{matrix}{{f(x)} = {\frac{1}{\sqrt{2\pi} \cdot \sigma}{\exp\left( {- \frac{\left( {x - \mu} \right)^{2}}{2\sigma^{2}}} \right)}}} & (1)\end{matrix}$

The frequency distribution generation unit 312 outputs the calculatedmean μ and standard deviation σ to a quantization thresholddetermination unit 313 as information indicating the frequencydistribution of the sampled values.

The quantization threshold determination unit 313 generates a cumulativedistribution function g(x) corresponding to the probability densityfunction f(x) indicated by equation (1), as given by:

$\begin{matrix}{{g(x)} = {\frac{1}{2}\left\{ {1 + {{erf}\left( \frac{x - \mu}{\sqrt{2} \cdot \sigma} \right)}} \right\}}} & (2)\end{matrix}$

where erf( ) represents an error function. The quantization thresholddetermination unit 313 determines a plurality of (2^(M)) quantizationthresholds based on the cumulative distribution function g(x) indicatedby equation (2). More specifically, the quantization thresholddetermination unit 313 determines, as a plurality of quantizationthresholds, a plurality of sampled values corresponding to a pluralityof thresholds for equally dividing a value range (that is, [0, 1])within which the cumulative distribution function g(x) can fall.

As described above, obtaining a plurality of values of x for equallydividing the value range within which the cumulative distributionfunction g(x) can fall is equivalent to obtaining a plurality of ranges(sections) of x within which the integrated values of the probabilitydensity function f(x) (corresponding to the frequencies of occurrence ofthe sampled values) are equal to each other. Therefore, by thusobtaining the plurality of values of x, it is possible to determine aplurality of quantization thresholds in accordance with the frequencydistribution as described above. That is, it is possible to determine aplurality of thresholds so that, for each of the plurality of sectionsobtained by dividing, using the plurality of thresholds, a range withinwhich sampled values can fall, the width of the section becomes narroweras the frequency of occurrence of the sampled value included in thesection is higher. Furthermore, it is possible to determine theplurality of thresholds so that, for each section, the width of thesection becomes wider as the frequency of occurrence of the sampledvalue included in the section is lower.

A process of determining a plurality of quantization thresholds by thequantization threshold determination unit 313 will now be described inmore detail with reference to FIG. 9. As denoted by reference numeral900 in FIG. 9, the frequency distribution generation unit 312approximates an actual frequency distribution 901 by a normaldistribution 902 (N(μ, σ²)) by calculating the mean μ and standarddeviation σ of the frequencies of occurrence of the sampled values.

Next, as denoted by reference numeral 910 in FIG. 9, the quantizationthreshold determination unit 313 multiplies the cumulative distributionfunction g(x) by the number 2^(M) of quantization thresholds to bedetermined, thereby scaling g(x) by 2^(M). Consequently, a value rangewithin which the scaled cumulative distribution function g(x) can fallis obtained as [0, 2^(M)]. As a result, it is possible to obtain aplurality of sampled values corresponding to a plurality of thresholdsfor equally dividing the value range within which the cumulativedistribution function can fall, by obtaining the sampled values x withwhich g(x)×2^(M) yields natural numbers (1, 2, . . . , and 2^(M)).

Note that the sampled values x are discrete values each represented by Nbits, and thus it may be impossible to strictly obtain the sampledvalues x with which g(x)×2^(M) yields natural numbers (1, 2, . . . , and2^(M)). Therefore, the quantization threshold determination unit 313 mayspecify 2^(M) sampled values x with which g(x)×2^(M) yields valuesclosest to 2^(M) natural numbers (1, 2, . . . , and 2 ^(M)),respectively, and determine them as 2^(M) quantization thresholds. Morespecifically, a value closest to g(x)×2^(M)=j(j=1, 2, . . . , 2^(M))among the possible sampled values x indicated by N-bit sampled data isdetermined as the j-th quantization threshold (920 in FIG. 9).

With this process, it is possible to determine a plurality ofquantization thresholds so that the integrated value, in a sectionbetween the j-th quantization threshold and a (j+1)-th quantizationthreshold, of the normal distribution N(μ, σ²) corresponding to thefrequencies of occurrence of the sampled values becomes equal for eachof j=1, 2, . . . , 2^(M)−1 (as much as possible). Note that thequantization threshold determination unit 313 generates data indicatinga conversion table as shown in FIG. 8 based on the plurality ofdetermined quantization thresholds, and outputs the resultant data to acoder 314 and a compression information addition unit 315, similarly tothe first embodiment.

By executing the same compression processing as in the first embodiment,the coder 314 compresses N-bit sampled data to M-bit sampled data andoutputs it. In addition, by executing the same processing as in thefirst embodiment, the compression information addition unit 315transmits, to the BBU 130 via a communication line 150, compressed dataadded with control information (compression information) to be used indecompression processing by a decompression unit 211 of the BBU 130.Note that the compression information addition unit 315 may transmit, asthe control information transmitted to the BBU 130, the mean μ andstandard deviation σ calculated by the frequency distribution generationunit 312 to the BBU 130.

<Decompression Unit>

By executing the same decompression processing as in the firstembodiment, the decompression unit 211 of the BBU 130 can decompressM-bit compressed data to N-bit sampled data. Note that if the mean μ andstandard deviation σ of the frequencies of occurrence of the sampledvalues are received from the RFU 120 as control information, aquantization threshold determination unit 412 determines, based on μ andσ, a plurality of quantization thresholds to be used in decompressionprocessing by a decoder 414. In this case, the quantization thresholddetermination unit 412 determines a plurality of (2^(M)) quantizationthresholds based on the cumulative distribution function g(x) indicatedby equation (2), and generates data indicating the conversion tableshown in FIG. 8, similarly to the quantization threshold determinationunit 313 on the side of the RFU 120.

By executing the same decompression processing as in the firstembodiment, the decoder 414 converts (decompresses) M-bit compresseddata to N-bit sampled data, and outputs it.

As described above, according to this embodiment, it is possible tocompress a baseband signal at a high compression ratio while suppressingdegradation in signal quality along with compression processing, byadaptively controlling quantization thresholds to be used for datacompression in accordance with the frequency distribution, similarly tothe first embodiment.

The present invention is not limited to the above embodiments andvarious changes and modifications can be made without departing from thespirit and scope of the present invention. Therefore, to apprise thepublic of the scope of the present invention, the following claims aremade.

1. A base station system comprising a baseband unit and a radiofrequency (RF) unit connected to the baseband unit via a communicationline, the RF unit comprising: a generation unit configured to generate,as sampled data, a digital signal corresponding to a baseband signal tobe transmitted to the baseband unit; a measurement unit configured tomeasure frequencies of occurrence of sampled values indicated by thesampled data generated by the generation unit, and generate a frequencydistribution representing a relationship between a sampled value havingoccurred and a frequency of occurrence; a determination unit configuredto determine a plurality of thresholds for compressing the sampled data,which are used for quantization processing of a sampled value, inaccordance with the frequency distribution generated by the measurementunit; and a compression unit configured to compress the sampled data bythe quantization processing using the plurality of thresholds determinedby the determination unit to generate compressed data to be transmittedto the baseband unit.
 2. The base station system according to claim 1,wherein the determination unit determines the plurality of thresholds sothat, for each of a plurality of sections obtained by dividing, usingthe plurality of thresholds, a value range within which the sampledvalues can fall, a width of the section becomes narrower as thefrequency of occurrence of the sampled value included in the section ishigher, and the width of the section becomes wider as the frequency ofoccurrence of the sampled value in the section is lower.
 3. The basestation system according to claim 1, wherein the determination unitdetermines the plurality of thresholds so that integrated values of thefrequencies of occurrence of the sampled values in respective ones of aplurality of sections obtained by dividing, using the plurality ofthresholds, a value range within which the sampled values can fall,become equal with respect to the plurality of sections.
 4. The basestation system according to claim 1, wherein the measurement unitcalculates a mean and standard deviation σf the frequencies ofoccurrence from the measured frequencies of occurrence of the sampledvalues, and generates, as the frequency distribution, a normaldistribution determined by the calculated mean and standard deviation.5. The base station system according to claim 4, wherein thedetermination unit generates a cumulative distribution function of thenormal distribution generated as the frequency distribution, anddetermines the plurality of thresholds based on the generated cumulativedistribution function.
 6. The base station system according to claim 5,wherein the determination unit determines, as the plurality ofthresholds to be used for the quantization processing, a plurality ofsampled values corresponding to a plurality of thresholds for equallydividing a value range within which the cumulative distribution functioncan fall.
 7. The base station system according to claim 1, wherein theRF unit further comprises a transmission unit configured to transmit thecompressed data to the baseband unit together with control informationindicating the plurality of thresholds determined by the determinationunit, and the baseband unit comprises a decompression unit configured toreceive the control information and the compressed data transmitted bythe transmission unit, and decompress the compressed data using theplurality of thresholds indicated by the control information.
 8. Thebase station system according to claim 4, wherein the RF unit furthercomprises a transmission unit configured to transmit the compressed datato the baseband unit together with control information indicating theunit and standard deviation calculated by the measurement unit, and thebaseband unit comprises a decompression unit configured to receive thecontrol information and the compressed data transmitted by thetransmission unit, and decompress the compressed data using theplurality of thresholds determined based on the mean and standarddeviation indicated by the control information.
 9. The base stationsystem according to claim 1, wherein the generation unit generates, asthe sampled data, data of a first number of bits quantized using aplurality of thresholds with equal intervals, and the determination unitdetermines a plurality of thresholds with unequal intervals forcompressing the sampled data to data of a second number of bits smallerthan the first number of bits.
 10. The base station system according toclaim 1, wherein the sampled data includes in-phase (I) channel data andquadrature (Q) channel data, and the measurement unit, the determinationunit, and the compression unit individually execute processing for eachof the I channel data and the Q channel data which are included in thesampled data.
 11. The base station system according to claim 1, whereinthe baseband unit and the RF unit are connected via an optical line. 12.The base station system according to claim 1, wherein the generationunit generates the sampled data by down-converting, to a basebandsignal, an analog signal received via an antenna connected to the RFunit, and converting the baseband signal into a digital signal.
 13. Abase station system comprising a baseband unit and a radio frequency(RF) unit connected to the baseband unit via a communication line, thebaseband unit comprising: a generation unit configured to generate, assampled data, a digital signal corresponding to a baseband signal to betransmitted to the RF unit; a measurement unit configured to measurefrequencies of occurrence of sampled values indicated by the sampleddata generated by the generation unit, and generate a frequencydistribution representing a relationship between a sampled value havingoccurred and a frequency of occurrence; a determination unit configuredto determine a plurality of thresholds for compressing the sampled data,which are used for quantization processing of a sampled value, inaccordance with the frequency distribution generated by the measurementunit; and a compression unit configured to compress the sampled data bythe quantization processing using the plurality of thresholds determinedby the determination unit to generate compressed data to be transmittedto the RF unit.
 14. A communication apparatus for communicating with anopposing apparatus connected via a communication line in a base stationsystem, comprising: a generation unit configured to generate, as sampleddata, a digital signal corresponding to a baseband signal to betransmitted to the opposing apparatus; a measurement unit configured tomeasure frequencies of occurrence of sampled values indicated by thesampled data generated by the generation unit, and generate a frequencydistribution representing a relationship between a sampled value havingoccurred and a frequency of occurrence; a determination unit configuredto determine a plurality of thresholds for compressing the sampled data,which are used for quantization processing of a sampled value, inaccordance with the frequency distribution generated by the measurementunit; and a compression unit configured to compress the sampled data bythe quantization processing using the plurality of thresholds determinedby the determination unit to generate compressed data to be transmittedto the opposing apparatus.
 15. The communication apparatus according toclaim 14, wherein the communication apparatus is a radio frequency (RF)unit of the base station system, and the opposing apparatus is abaseband unit of the base station system.
 16. The communicationapparatus according to claim 14, wherein the communication apparatus isa baseband unit of the base station system, and the opposing apparatusis a radio frequency (RF) unit of the base station system.